1. FIELD OF THE INVENTION
This invention relates to low noise alternating current dynamoelectro machines wherein electromagnetic vibrations of the machine are suppressed, more particularly their core structures wherein electromagnetic vibrations caused in the rotor core or stator core by speed controlling with thyristors are reduced.
2. DESCRIPTION OF THE PRIOR ART
Recently, for AC motors generally exemplified by single-phase and three-phase induction motors, speed controlling with thyristors has widely been employed. Especially, AC motors used for elevators are required to rotate smoothly independent of the magnitude of load under a given acceleration and deceleration. For this reason, upon speed controlling, a DC current obtained by full-wave rectification of an AC current is passed through the field winding of AC motor and the intensity of a magnetic field induced from the DC current is adjusted to control the rotation of the AC motor. Average value of the DC current flow through the field winding is adjusted by controlling the ignition angle of thyristors.
However, a high frequency current produced when the ignition angle of thyristors is controlled induces a high frequency distortion in the magnetic flux between the rotor and stator, and an electromagnetic force due to the magnetic flux distortion causes electromagnetic vibrations and noises in the AC motor.
The electromagnetic force wave due to the high frequency magnetic flux between the rotor and stator varies at a frequency as a function of the slip of the AC motor. Therefore, when the natural frequency of the rotor core or stator core coincides with frequency of the electromagnetic force wave, a resonance is caused with the result that the vibrations and noises are accelerated.
Specifically, since AC motors such as for elevators undergo a frequent start-stop operation and are installed indoors, it is an important problem for them to solve that vibrations are reduced.
A conventional measure to prevent such electromagnetic vibrations was either to increase a gap between the rotor and stator or to let a magnetic circuit through the rotor and stator have more allowance than that practically required. The measure, however, was disadvantageous in that not only efficiency of the AC motor was degraded but also size and weight thereof were increased.
In addition, the rotor core, stator core and a support structure therefor were so designed under calculation of their natural frequencies that their natural frequencies were not coincident with a frequency of the electromagnetic force wave. The calculation of natural frequencies of the rotor core, stator core and the support structure therefor, however, suffers from indefinite factors mainly depending on coupling conditions of the support structure and a highly precise calculation of these natural frequencies was not expected. On the other hand, the frequency of the electromagnetic force wave will vary with supply voltage distortion due to the speed controlling with thyristors, and start and deceleration of the AC motor as well as slip fluctuations due to pole change.